1. Field of the Invention
The present invention relates to an electrophotographic toner and a method for producing a toner, and to a developer and an image forming apparatus that contain the toner.
2. Description of the Related Art
In recent years, demand has arisen on the market for toners having various advantageous properties such as small particle diameters for forming high-quality output images and improved low-temperature fixing property for energy saving.
Toners obtained by the conventional kneading-pulverizing method are not easily made to have a small particle diameter. In addition, their shape is amorphous and their particle size distribution is broad. Furthermore, these toners have various problems such as requiring a large amount of energy for being fixed.
In particular, a toner produced by the kneading-pulverizing method causes a problem related to fixing. Specifically, cracks occur at the interfaces of releasing agent (wax) during pulverization, resulting in that the wax exists on the surface of the toner particle in a large amount.
As a result, although the releasing effects can be obtained, toner adhesion to a carrier, a photoconductor and a blade is likely to occur. The properties of such toners are not satisfactory in total.
In order to overcome the above-described problems the kneading-pulverizing method has, there is proposed a method for producing a toner by the polymerization method.
According to the polymerization method, toners are made easily to have a small particle diameter. Their particle size distribution is sharper than that of the toners obtained by the pulverizing method. Furthermore, the wax can be embedded in the toner particles.
As a toner production method based on the polymerization method, Japanese Patent Application Laid-Open (JP-A) No. 11-133665 discloses a method for producing a toner having a practical sphericity of 0.90 to 1.00. In this method, the toner is produced from an elongation reaction product of a urethane-modified polyester serving as a toner binder, for the purpose of improving the toner in flowability, low-temperature fixing property and hot offset resistance.
Besides, for example, JP-A Nos. 2002-287400 and 2002-351143 disclose a method for producing a toner excellent in all of heat resistance storage stability, low-temperature fixing property and hot offset resistance. The toner produced with this method has a small particle diameter to be excellent in flowability as powder and transferability.
The toner production methods disclosed in JP-A Nos. 11-133665, 2002-287400 and 2002-351143 each include a step of allowing a polyester resin, and if necessary, an isocyanate group-containing polyester prepolymer to undergo polymerization addition reaction with an amine in the reaction system containing an organic solvent and an aqueous medium to increase the molecular weight.
However, the toner obtained by the above-described method is increased in hot offset resistance but degraded in low-temperature fixing property and in glossiness after fixing. This is because toner particles are produced in the aqueous medium and thus materials with high polarity such as a pigment cannot be retained in the toner particle and the materials with high polarity tend to locate on the surface of the toner particle. Therefore, the above-described method is still not satisfactory.
When an isocyanate group-containing polyester prepolymer is used, a large number of nitrogen atoms are located on the surfaces of the toner particles and produced polymers have positive polarity. Thus, use of the isocyanate group-containing polyester prepolymer is not suitable for the electrophotographic toner of negative polarity.
In addition, there has been disclosed a method for producing a toner binder including an aging step for producing a toner binder having a stable molecular weight distribution and achieving a favorable balance between low-temperature fixing property and hot offset resistance (see, for example, Japanese Patent (JP-B) No. 2579150 and JP-A No. 2001-158819).
The production methods of the toner binder disclosed in JP-B No. 2579150 and JP-A No. 2001-158819 are easily applicable to condensation polymerization reaction which is performed at a high temperature. But, they are not applicable to the above-described reaction system, which contains an organic solvent and an aqueous medium, without conducting studies on suitable conditions. Thus, it is difficult to obtain a toner that contains wax embedded in the toner particles and that has a sharp particle size distribution.